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Commit 1f558098 authored by Marc Zyngier's avatar Marc Zyngier Committed by Christoffer Dall
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ARM: KVM: Yield CPU when vcpu executes a WFE 



On an (even slightly) oversubscribed system, spinlocks are quickly
becoming a bottleneck, as some vcpus are spinning, waiting for a
lock to be released, while the vcpu holding the lock may not be
running at all.

This creates contention, and the observed slowdown is 40x for
hackbench. No, this isn't a typo.

The solution is to trap blocking WFEs and tell KVM that we're
now spinning. This ensures that other vpus will get a scheduling
boost, allowing the lock to be released more quickly. Also, using
CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the performance
when the VM is severely overcommited.

Quick test to estimate the performance: hackbench 1 process 1000

2xA15 host (baseline):	1.843s

2xA15 guest w/o patch:	2.083s
4xA15 guest w/o patch:	80.212s
8xA15 guest w/o patch:	Could not be bothered to find out

2xA15 guest w/ patch:	2.102s
4xA15 guest w/ patch:	3.205s
8xA15 guest w/ patch:	6.887s

So we go from a 40x degradation to 1.5x in the 2x overcommit case,
which is vaguely more acceptable.

Signed-off-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
parent a7265fb1

arch/arm/include/asm/kvm_arm.h

+3 −1
Original line number Diff line number Diff line
@@ -67,7 +67,7 @@ 
 */

#define HCR_GUEST_MASK (HCR_TSC | HCR_TSW | HCR_TWI | HCR_VM | HCR_BSU_IS | \

			HCR_FB | HCR_TAC | HCR_AMO | HCR_IMO | HCR_FMO | \

			HCR_SWIO | HCR_TIDCP)

			HCR_TWE | HCR_SWIO | HCR_TIDCP)

#define HCR_VIRT_EXCP_MASK (HCR_VA | HCR_VI | HCR_VF)



/* System Control Register (SCTLR) bits */
@@ -208,6 +208,8 @@ 
#define HSR_EC_DABT	(0x24)

#define HSR_EC_DABT_HYP	(0x25)



#define HSR_WFI_IS_WFE		(1U << 0)



#define HSR_HVC_IMM_MASK	((1UL << 16) - 1)



#define HSR_DABT_S1PTW		(1U << 7)

arch/arm/kvm/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -20,6 +20,7 @@ config KVM 
	bool "Kernel-based Virtual Machine (KVM) support"

	select PREEMPT_NOTIFIERS

	select ANON_INODES

	select HAVE_KVM_CPU_RELAX_INTERCEPT

	select KVM_MMIO

	select KVM_ARM_HOST

	depends on ARM_VIRT_EXT && ARM_LPAE

arch/arm/kvm/handle_exit.c

+5 −1
Original line number Diff line number Diff line
@@ -84,7 +84,11 @@ static int handle_dabt_hyp(struct kvm_vcpu *vcpu, struct kvm_run *run) 
static int kvm_handle_wfi(struct kvm_vcpu *vcpu, struct kvm_run *run)

{

	trace_kvm_wfi(*vcpu_pc(vcpu));

	if (kvm_vcpu_get_hsr(vcpu) & HSR_WFI_IS_WFE)

		kvm_vcpu_on_spin(vcpu);

	else

		kvm_vcpu_block(vcpu);



	return 1;

}